Recently, as electronic appliances have been downsized and lightened, it has been required to downsize and lighten batteries serving as power sources for electronic appliances. As a compact, low-weight and high-capacity rechargeable battery, a lithium secondary battery has been commercialized and used widely in portable electronic and communication instruments, such as compact video cameras, portable phones, notebook computers, etc.
A lithium secondary battery comprises a cathode, an anode, a separator and a non-aqueous electrolyte containing an electrolyte salt and an electrolyte solvent.
With regard to the operation and use of a battery, the non-aqueous electrolyte is required to have the following characteristics. First, the non-aqueous electrolyte should serve to transfer lithium ions between the cathode and the anode upon the lithium ion intercalation/deintercalation in the two electrodes. Next, the non-aqueous electrolyte should be electrochemically stable under the potential difference between the cathode and the anode and have little possibility of side reactions, such as the decomposition of the electrolyte.
However, an electrode comprising a carbonaceous material and another electrode formed of a lithium metal compound, generally used as an anode and a cathode for a battery, show a difference in the potentials of about 3.5˜4.3V. Under the potential difference, a conventional electrolyte solvent, such as a carbonate-based organic solvent, may be decomposed on the surface of an electrode during repeated charge/discharge cycles, thereby causing a side reaction inside the battery. Additionally, organic solvents, such as propylene carbonate (PC), dimethyl carbonate (DMC) or diethyl carbonate (DEC) may be co-intercalated into a gap between graphite layers of an anode comprising a carbonaceous material, resulting in a structural collapse of the anode.
Meanwhile, it has been known that the above problems could be solved by a solid electrolyte interface (SEI) layer formed on the surface of an anode via the electrical reduction of a carbonate-based organic solvent upon the initial charge of the battery. However, lithium ions in the electrolyte irreversibly participate in the formation of the SEI layer, resulting in a drop in the capacity of the battery. Particularly, the SEI layer is not electrochemically or thermally stable, and thus may be easily broken down by electrochemical energy and heat energy increasing with the lapse of time during repeated charge/discharge cycles. Therefore, the battery may show a drop of the capacity during repeated charge/discharge cycles due to the continuous regeneration of the SEI layer, and may undergo degradation of its lifespan characteristics.
Further, side reactions, such as the decomposition of the electrolyte, may occur on the surface of the anode exposed due to the collapse of the SEI layer, and the gases generated upon the side reactions result in a battery swelling phenomenon or an increase in the internal pressure of the battery.
To solve the aforementioned problems, methods of adding 1,3-propanesultone (Japanese Patent Application No. 1999-339850) or 1,3-propenesultone (Japanese Patent Application No. 2001-151863) to an electrolyte have been suggested. However, even when applying such methods, batteries undergo a gradual drop in their capacities as they are subjected to charge/discharge cycles continuously. Therefore, the aforementioned problems still remain unsolved.